This invention relates to an apparatus for measuring radiation dose.
The effects of irradiation of ionizing radiations such as electron beams and gamma rays are widely used in industry. For example, they are used in the development of polymers, and the technique of irradiating foods with such ionizing radiations for the purpose of, say, preventing the germination of potatoes is used on an commercial scale. Active effors are being made in other fields such as air and water pollution controls, as well as the production of semiconductors having improved characteristics. In the treatment with ionizing radiations, it is necessary to know the absorbed dose, i.e. the amount of the radiation energy absorbed per. unit mass in the material irradiated. There are many methods to determine the absorbed dose, and the method of using the change in light absorbance upon irradiation is widely used partly because it is simpler to use than the others. Among dosimeters used in this method are a cellulosetriacetate (CTA) film dosimeter, polyethylene terphthalate (PET) film dosimeter, polymethylmethacrylate (PMMA) dosimeter, polycarbonate dosimeter, cobalt glass dosimetry, blue Cellophane dosimeter, and a radiochromic dosimetry materials.
FIG. 1 shows the absorption spectrum of an unirradiated sample and an electron beam irradiated sample (5.5 Mrad) as obtained by a CTA dosimeter at wavelength of 270 to 340 nm. Apparently, there occurs a change in light absorbance in the stated range of wavelengths, and readings of light absorbance are usually taken at 280 nm. The relation between the change in light absorbance at 280 nm and the absorbed dose is shown in FIG. 2 from which one can see that the change in light absorbance is linear to an absorbed dose of up to 15 Mrad. Therefore, change in light absorbance can be easily converted to absorbed dose using a calibration factor.
The light absorbance is usually measured with a spectrophotometer for ultraviolet and visible ranges that uses diffraction grating and/or a prism to perform the spectrophotometry of a light from a hydrogen discharge tube and a tungsten lamp. The spectrophotometer is capable of determining the absorption spectrum but it has the following disadvantages: (1) it is not portable and is not handy to use; (2) it is subject to false wavelength setting (for a CTA dosimeter, an error in wavelength setting of .+-.0.3 nm results in an error in dose measurement of 2%); (3) the light source and the reading wavelength must be set and the slit width adjusted for each type of dosimeter and hence, the spectrophotometer is not simple to use; and (4) the equipment is generally expensive. A photometer that is intended to eliminate these defects by using an optical filter to provide a narrow band of wavelengths is commercially available, but its power to resolve wavelengths is so low that precise measurement is not expected.
Therefore, one purpose of this invention is to provide a new apparatus for measuring absorbed dose that is free from the defects of the spectrophotometer and other conventional products. We made various studies to attain this purpose and the primary objective of the studies was to develop an accurate and simple means to obtain a light of a monochromatic light consistently. This objective is best attained by a light source that emits monochromatic light of reading wavelength without spectrophotometric system and hence the light source must have line spectrum but not continuous spectrum. To eliminate these problems, we came to think of the use of a hollow cathode lamp as the light source.